Kelly Osbourne, Justin Bieber, Wilmer Valderrama, Lady Gaga, Ellen DeGeneres, Ariana Grande, and Brad Paisley are among those who have taken to social media to pen well wishes to the Sober and Confident artist.

During an appearance on British talk show Loose Women, Osbourne said she knows exactly what Lovato is going through because she too has battled drug addiction.

"Look, I will never speak on behalf of Demi, because that wouldn't be right. I can only share about what I've been through and what I know from myself. And that is, relapse is one of the hardest things we face as an open addict," she said.

Osbourne, 33, wants people to understand that rehab isn't a quick fix -- every day is a struggle for people like herself and Lovato.

"People need to know rehab doesn't fix you, it just helps you to get yourself on the right track," added Osbourne. "You're never fixed. You spend your whole entire life doing everything you can to never pick up and use again."

Bieber was also stunned to hear about Lovato's relapse. On Wednesday, a photographer asked if he'd spoken to the singer. He said he hasn't but that the situation is "very sad."

"My prayers go out to her and her family for sure," Bieber said, in video obtained by TMZ. "I thought she was sober. That's terrible, right?"

Valderrama, Lovato's ex-boyfriend, was spotted visiting the singer in hospital the day after her hospitalisation. According to E! News, he spent two hours with her.

"He was sombre and quiet and you could tell he was in a very serious mood. He arrived and left by himself," a source told the outlet.

"He knew she was going through a tough time but he wasn’t prepared for his," the mag's source said. "He saw her through so many ups and downs and was her rock through some of her darkest moments. To see her back in such a sad and vulnerable place is heartbreaking for him."

The pair dated for six years before announcing their split in June 2016.

Lady Gaga sent Lovato love on Twitter.

"We should all wrap our arms of love around Demi Lovato," she tweeted. "I am so happy you’re alive. Thank God. If I know my monsters as well as I believe I do, we all wish you self-compassion and inner peace. And may you receive the love so many have for you. #ImConfidentInDemi Demi, I love you."

"I love Demi Lovato so much," DeGeneres wrote on Twitter. "It breaks my heart that she is going through this. She is a light in this world, and I am sending my love to her and her family."

Lovato has been open about her personal struggles with eating disorders, body image and addiction through her music and documentaries. Paisley also praised his friend for her kindness and talent, as well as expanding his comments to address addiction in general, which he called "a terrifying disease."

"My friend Demi Lovato is one of the kindest, most talented people I've ever met," Paisley wrote. "Praying for her right now, addiction is a terrifying disease. There is no one more honest or brave than this woman."

Ariana Grande, who launched her star on Nickelodeon around the same time Lovato found fame at Disney Channel, wrote simply: "i love u," while Lovato's Camp Rock co-star Alyson Stoner and Even Stevens actress Christy Carlson Romano were among Lovato's Disney peers hoping for her recovery.

"Only love and support," Stoner wrote in a tweet tagging Lovato. "Save the rest."

Riverdale star Lili Reinhart detailed her personal connection to Lovato, as well as the singer and activist's outspokenness on mental health and body image.

"Praying for Demi Lovato and her health," Reinhart wrote. "When I was 14, she was an idol to me in how she spoke so openly about mental health. And now she continues to inspire thousands young women with her body positivity messages. Addiction and mental illness doesn't discriminate."